The present invention relates to a system and method for determining the location of emitters.
Determining the location of emitters of electromagnetic (xe2x80x9cEMxe2x80x9d) radiation in a monitored area, especially multiple emitters in an area, is a matter of interest in many applications, such as military or security applications. In one such application, it is desirable to determine the location of EM emitters in a monitored geographic area. Known methods for determining the location of EM emitters involve collecting electromagnetic radiation data simultaneously at a plurality of EM receiver platform sites having known locations in the monitored area. The EM data collected at the EM receiver platforms may be filtered in order to isolate a particular frequency range of interest.
Known methods determine the time difference of arrival (xe2x80x9cTDOAxe2x80x9d) and/or frequency difference of arrival (xe2x80x9cFDOAxe2x80x9d) of the electromagnetic radiation data collected at the EM receiver platforms. TDOA relates to the time shift in receiving the EM data at the various EM receiver platforms. TDOA results from, among other things, differences in the signal path length between the emitters and receivers and differences in signal propagation mediums. FDOA relates to a frequency shift or Doppler shift in the EM data received at the various EM receiver platforms. FDOA results from, among other things, movement of the emitters and/or receivers.
In the methods set forth above, TDOA and/or FDOA data collected at two EM receiver platforms is used to generate a three-dimensional contour representative of the EM radiation in the monitored area of interest. The number of generated contours can be increased by increasing the number of EM receivers used to collect the data. For example, if there are three EM receivers (e.g., receivers A, B, and C), a first surface can be generated using TDOA/FDOA data collected at receivers A and B, a second surface can be generated using TDOA/FDOA data collected at receivers B and C, and a third surface can be generated using TDOA/FDOA data collected at receivers A and C.
In the contours, time shift (xcfx84) and frequency shift (xcexd) are typically represented on the horizontal axes and the magnitude of the detected electromagnetic radiation is represented on the vertical axis. If there is no FDOA (i.e., if the emitters and receivers are stationary), then the frequency shift (xcexd) is zero. xe2x80x9cPeaksxe2x80x9d on the contour having an EM radiation level above a predetermined threshold are determined to be indicative of EM emitters. The predetermined threshold is determined as a function of electromagnetic noise levels in the geographic area of interest. Electromagnetic noise is a function of a variety of factors, such as the number of EM emitters in the monitored area of interest.
The peaks on the contour are located using known mathematical routines. Once a peak is found, the TDOA/FDOA information relating to the peaks can be determined from the contour and recorded. Thus, in the example set forth above, TDOA/FDOA data for potential EM emitters is determined from each of the three generated contours.
In order to determine the location of the EM emitters, the TDOA/FDOA data for each contour is reduced to two-dimensions wherein XY positions are mapped to correspond to the monitored area of interest (e.g., latitude and longitude). The TDOA/FDOA data recorded from the contours is used to calculate curves indicative of constant time and/or frequency paths for each potential emitter. If there is no frequency shift (i.e., FDOA=0), the curve is purely a constant time path curve, which takes the form of a hyperbola.
In the example set forth above, three sets of curves (one for each of contours A-B, B-C, and C-A) would be calculated. In each set of curves, there would be a constant time/frequency path curve for each EM emitter. In order to determine the EM emitter locations, curves calculated using data from two of the contours (e.g., contours A-B and B-C) are compared mathematically to determine where the curves intersect.
Locations where the curves for a particular EM emitter intersect each other indicate potential EM emitter locations. However, when multiple EM emitters are encountered, xe2x80x9cghostxe2x80x9d locations are generated in addition to the real EM emitter locations. Ghost locations occur where curves intersect but no EM emitter is located. This occurs when the curve for a particular EM emitter generated by one receiver pair intersects a curve for a different EM emitter that is generated by another receiver pair. It will be appreciated that the number of ghost locations increases exponentially as the number of EM emitters in the monitored area increases.
In order to eliminate ghost locations, the intersections determined from the first two contours (A-B and B-C) are compared with the curves generated from the remaining contour (contour C-A). Locations where the curves generated from contour C-A intersect the intersections determined from contours A-B and B-C help isolate the xe2x80x9crealxe2x80x9d EM emitter locations from the ghost locations.
The mathematical calculations used to determine curve intersections can become extremely complex, especially where there is both a time shift (xcfx84) and a frequency shift (xcexd) in the EM data. Increases in the number of EM emitters and, thus, noise in the monitored area also increases the mathematical complexity of the operation. Such complexity is undesirable for obvious reasons, such as processing speed, system expense, etc.
The present invention is directed to a system and method for determining locations of multiple emitters. According to the present invention, the intersection of constant time and/or frequency path curves are determined by providing the curves in an array and systematically analyzing assigned attributes for the elements in the array to determine the location of any intersections. In an example embodiment of the present invention, the array comprises a graphical image array including a plurality of elements in the form of pixels. In the example embodiment, the intersections of the constant time and/or frequency path curves are determined graphically by analyzing the arrangement of the pixels in the graphical image.
The constant time and/or frequency path curves are determined from data collected by two or more different EM receiver pairs. The constant time and/or frequency path curves generated by each EM receiver pair are assigned an attribute, such as color, that is unique to that pair. The curves are provided in the graphical array with the unique attribute (color) being assigned to each pixel along the curve.
In the example embodiment of the present invention, the pixels in the graphical image are scanned systematically. An intersection is determined when a pixel having an assigned attribute (color) indicative of one EM receiver pair is positioned adjacent to a predetermined number of pixels having an assigned attribute (color) indicative of another EM receiver pair.
According to one aspect of the present invention, a method for determining a location of an emitter in a monitored area includes the step of providing an array representative of the monitored area. The array includes a plurality of elements. Next, at least one first and second curves are provided in the array. The first and second curves are representative of possible locations of the emitter in the monitored area. Then, possible emitter locations are identified at intersections of the first and second curves. The intersections are identified by determining locations in the array where an element having an assigned attribute corresponding to the first curve has a predetermined number of adjacent elements having an assigned attribute corresponding to the second curve.
According to another aspect of the present invention, a computer product is operative to determine a location of an emitter in a monitored area. The computer includes a portion for providing an array representative of the monitored area. The array includes a plurality of elements. The computer product includes another portion for providing at least one first and second curves in the array. The first and second curves are representative of possible locations of the emitter in the monitored area. Another portion of the computer product identifies possible emitter locations at intersections of the first and second curves. The portion identifies the intersections by determining locations in the array where an element having an assigned attribute corresponding to the first curve has a predetermined number of adjacent elements having an assigned attribute corresponding to the second curve.